The present invention relates to hydraulic oil compositions for shock absorbers.
There are various types of shock absorbers, but they basically comprise a piston with a valve attached thereto and an outer cylinder. The piston is fixed to a rod and slides up and down along the inner surface of the cylinder while the rod slides along the seal portion of a rod guide means. A shock absorber is usually filled with a hydraulic oil and if necessary gas and dampens shocks by the resistance of the oil to the valve movement.
Since in operation a hydraulic oil for shock absorber is always in an oscillation state and mixed with air or gas, the oil is likely to create air bubbles or foams. Rapid movement of the piston creates reduced pressure which also causes bubbling or foaming. Furthermore, the hydraulic oil for shock absorber is exposed to the outdoor air and the temperature of the oil also change from lower temperatures to elevated temperatures due to the change of the outdoor air temperature. As a result of such temperature change, the oil changes in viscosity, resulting in changes in the damping force of the shock absorber. In order to suppress the viscosity change as much as possible, a viscosity index improver has been used. However, it is known that addition of such a viscosity index improver is likely to create bubbles. Therefore, the hydraulic oil for shock absorbers is always exposed to situations under which bubbles are likely to be formed. When the valve moves through such bubbles, it meets little resistance and thus fails to generate any damping force. Therefore, the shock absorber can not absorb oscillation caused by road impacts or shocks or the like sufficiently and adversely affect the ride comfort of an automobile.
Conventionally, a shock absorber has been improved in friction characteristics, anti-wear properties or durability by optimizing a friction modifier or an anti-wear agent for a hydraulic oil for a shock absorber (for example, as disclosed in Japanese Patent Laid-Open Publication Nos. 7-224293, 7-258678, 6-128581, 2000-192067, 2002-194376 and 5-255683). Recently, it is reported that the ride comfort of an automobile is improved by enhancing the frictional force of a shock absorber (Japanese Patent Laid-Open Publication No. 2004-035624). In general, the compositions disclosed in the above-mentioned publications contain a polymethacrylate-based viscosity index improver. Alternatively, Japanese Patent Laid-Open Publication No. 2002-053886 discloses a hydraulic oil composition for shock absorbers containing an ethylene-propylene copolymer or a styrene-maleic acid ester copolymer in an amount of 1 to 15 percent by mass based on the mass of the resin, which copolymer is excellent in anti-cavitation properties and can provide a shock absorber with long-lasting damping force.
However, it is now found that mere selection of a proper friction modifier, anti-wear agent or viscosity index improver is insufficient to enhance the damping force of a shock absorber and improve a capability thereof to suppress an automobile from oscillating when it travels over obstacles such as bumpy road surfaces and absorb the oscillation or impacts caused thereby instantaneously (hereinafter referred to as “initial damping properties”) and thus the ride comfort of an automobile.